Heretofore, as a process for producing an alkali metal hydroxide through the electrolysis of an aqueous solution of an alkali metal salt, for example, a process for producing sodium hydroxide, chlorine and hydrogen through the electrolysis of an aqueous solution of sodium chloride, there have been known a process in which an anode chamber and a cathode chamber are demarcated by a cation exchange membrane, an anode is made existent in the anode chamber, a cathode is made existent in the cathode chamber, the anode chamber is filled with an aqueous solution of an alkali metal salt, the cathode chamber is filled with an aqueous solution of an alkali metal hydroxide, and DC current is applied between the both electrodes to carry out electrolysis as well as an ion exchange membrane electrolytic cell used in the process.
For the electrolysis of an aqueous solution of an alkali metal salt such as sodium chloride, a theoretical decomposition voltage is theoretically applied to obtain sodium hydroxide, chlorine and hydrogen equivalent to consumption power based on so-called “Faraday's law”. However, in general, the voltage between electrodes rises to produce a power loss due to the overvoltages of the electrodes, the electric resistance of the cation exchange membrane and the electric resistance of an aqueous solution of sodium chloride and an aqueous solution of sodium hydroxide existent between the electrodes.
Then, in order to reduce the power loss, various attempts have been made to reduce the distance between the electrodes. JP-B 5-34434, JP-B 63-53272 and JP-B 57-85981 propose a so-called “zero-gap electrolytic cell” in which at least one of an anode and a cathode is pressed against the other electrode together with a cation exchange membrane by a spring material, an elastic mat material or a spring to be brought into close contact with it. The present invention can provide a cathode which is advantageously used in this zero-gap electrolytic cell, that is, an electrolytic cell in which the anode and the cathode are opposed to each other in such a manner that they sandwich the cation exchange membrane or they are opposed to each other through the cation exchange membrane with a small space there between.
Technical development has been made on the cation exchange membrane, and a membrane capable of electrolysis at a high current efficiency and a low voltage, that is, a membrane capable of operation at a low electric power consumption rate has been developed.
Meanwhile, as for the electrodes, a so-called “dimensionally stable anode (DSA)” which is obtained by coating the surface of a conductive material having resistance to an anode chamber liquid, such as a titanium material, with a platinum group metal, an oxide thereof or a mixture of one of these substances and an oxide of the group IV metal of the periodic table has been developed as the anode.
A conductive substrate such as a soft steel or nickel substrate has been commonly used in the cathode but there are proposed various so-called “active cathodes” which are obtained by coating the surface of the conductive substrate with a metal or an alloy to reduce hydrogen overvoltage. We have proposed an active cathode obtained by electroplating a conductive substrate made of soft steel or nickel with a nickel-tin alloy containing 25 to 99 wt % of nickel and 75 to 1 wt % of tin. The above active cathode has a hydrogen overvoltage 0.2 to 0.3 V lower than that of soft steel or nickel, thereby greatly reducing the electrolysis voltage (refer to JP-B 63-4920).
However, according to our subsequent studies, it was found that, when electrolysis was carried out continuously by using the active cathode electroplated with a nickel-tin alloy, the current efficiency dropped and the electrolysis voltage rose, that is, the deterioration of electrolytic performance was observed as days pass after energization. It was also found that the above active cathode had a bad influence upon the performance of an ion exchange membrane as nickel and tin on the surface of the active cathode liquated out and were introduced into the ion exchange membrane during the period from the injection of a liquid to energization after the ion exchange membrane was set in the electrolytic cell, thereby deteriorating electrolytic performance.
Meanwhile, after a surface coating layer is formed by a method such as plating, the active cathode for electrolysis is often washed with an alkali aqueous solution to remove an organic substance and an alkali-soluble component (refer to JP-A 59-25986, JP-A 2000-144470, Japanese Patent No. 3624394 and Japanese Patent No. 3867913).
These patent documents disclose only the use of an aqueous solution of an alkali hydroxide such as sodium hydroxide as the alkali aqueous solution.
However, according to studies conducted by the inventors of the present invention, it was found that, when the coating material for the active cathode is a nickel-tin alloy, if the active cathode is washed with an aqueous solution of an alkali hydroxide, soluble tin can be removed but not nickel and the deterioration of the performance of the ion exchange membrane cannot be avoided.
Therefore, even when the surface coating material for the conductive substrate is a nickel-tin alloy, a process in which soluble tin and nickel can be removed without fail and therefore excellent electrolysis can be carried out for a long time at a high efficiency or a low voltage without deteriorating the performance of an ion exchange membrane has been desired.